This invention pertains to the conversion of cyclohexanol to .epsilon.-caprolactone and more particularly to the stepwise autoxidation of cyclohexanol followed by conversion of the oxidation product to .epsilon.-caprolactone.
Lactones are used directly for the synthesis of polyesters, polyurethanes, and the like and indirectly for the production of lactams which are in turn converted to polyamides. Epsilon-caprolactone is a prime example of this class of lactones serving as a starting material for the production of poly-(epsilon-caprolactones), polyurethane elastomers and polyamides, such as, nylon-6. The synthesis of lactones in general and epsilon-caprolactone in particular has therefore been the subject of many research efforts.
The oxidation of cyclic ketones to cyclic lactones was discovered in 1899 and has since been known as the Baeyer-Villiger Reaction, although it has been broadened to include acyclic ketones. The oxidizing agents used in this reaction have included permonosulfuric acid (Caro's Acid), perbenzoic acid, monoperphthalic acid, peracetic acid and trifluoroperacetic acid. The selenium dioxide-catalyzed reaction of hydrogen peroxide with cyclic ketones to afford ring-contracted carboxylic acids was first reported in 1957 by G. B. Payne, et al., (J. Org. Chem., 22, 1680, 1957). In 1959 H. M. Hellman et al. reported in Tetrahedron Letters, 1 (1959) the formation of Baeyer-Villiger products, that is, cyclic lactones by employing selenic acid (H.sub.2 SeO.sub.4) rather than selenium dioxide as the catalyst. However, they also obtained the ring-contracted carboxylic acids. Subsequent efforts to oxidize cyclic ketones up till the present time have afforded the same mixtures of cyclic ketones and ring-contracted carboxylic acid.
Commerically one of the most important applications of the oxidation of cyclic ketones to cyclic lactones is that of the conversion of cyclohexanone to epsilon-caprolactone. Japanese patent 6,910,243 (Chem Abstracts, 71, 60755a 1969) discloses the Baeyer-Villiger oxidation of cyclohexanone to epsilon-caprolactone using 30 percent aqueous hydrogen peroxide catalyzed by arsenic trioxide. An efficiency based on hydrogen peroxide of 74% was obtained and a conversion to the lactone of 64 percent. The Japanese patent No. 7007549 (Chemical Abstracts, 73, 14219x, 1970) describes the selenious acid (H.sub.2 SeO.sub.3)-catalyzed oxidation of cyclohexanone with hydrogen peroxide at low temperatures, that is, less than 23.degree. C. An efficiency of 11.2 percent to caprolactone based on 30 percent aqueous hydrogen peroxide was described together with a selectivity of lactone production of 58 percent.
It is believed that there are basically three methods of effecting the conversion of cyclohexanone to epsilon-caprolactone on a commercial scale. These are the direct oxidation of cyclohexanone with peracetic acid using ethyl acetate or acetone described in U.S. Pat. No. 3,522,279; the co-oxidation of aldehyde and cyclohexanone described in several references including Netherlands patent application No. 6,409,489 (Chemical Abstracts, 63, 8208f, 1965); and the oxidation of cyclohexanone with t-butyl hydroperoxide, catalyzed by boric anhydride, described in German Offenlegungschrift No. 2,253,963 (Chemical Abstract, 79, 456r, 1973).
While the above three described methods are commercial, they suffer in common the disadvantage of organic by-product formation. The economics of these processes are also dependent on the relative costs of the oxidants and their reduction products, viz., carboxylic acid from the first and second methods and t-butyl alcohol from the third method. This is especially significant in the case of the second method which at best produces nearly two moles of carboxylic acid per mole of epsilon-caprolactone.
Two variations of the first method described above have also been considered commercially, viz., the oxidation of cyclohexanone with peracetic acid using water as a solvent (Netherlands application No. 6,613,409; Chemical Abstract 67, 63845h, 1967) and the oxidation of cyclohexanone with aqueous performic acid (French patent 1,385,557; Chem. Abstracts, 62, 13051e, 1965).
These methods however also suffer the disadvantage of requiring the recycle of large amounts of carboxylic acids and the use of concentrated hydrogen peroxide.
There is therefore still a continuing need in this art for a method of synthesizing epsilon-caprolactone with a minimum of by-product formation.